The present invention relates to an ultrasonic machine having an amplitude control unit which controls at an amplitude value of an ultrasonic vibration unit provided in the ultrasonic machine.
Industrial applications of ultrasonic waves cover a wide range of fields. For instance, ultrasonic abrasive grain machining and ultrasonic cutting are quite well-known. Abrasive grain machining for boring, cutting, and polishing hard, brittle materials such as glass and silicon, includes the steps of ultrasonically elastic-vibrating a tool, pressing the ultrasonically-vibrated tool onto the work via a machining liquid containing abrasive grains, and gradually machining the work to the tool shape. In cutting, on the other hand, a tool such as a turning tool is given vibration in the cutting direction, thus performing predetermined machining on the work.
As illustrated in FIG. 3, the basic composition of such an ultrasonic machine utilized for carrying out the above machining includes a mechanical vibration unit, a work table 20 upon which a work 18 is placed, a feed unit 22 for feeding the work table 20 in X and Y directions, and an abrasive grain supply unit 24 provided if necessary. The mechanical vibration unit comprises a high frequency oscillator 58, a piezoelectric transducer 12 as an ultrasonic vibrator for converting high-frequency electric energy generated by the high frequency oscillator 58 into mechanical vibration energy, and a solid horn 16 for transmitting the vibration energy to a tool 14. The high-frequency oscillation circuit provided in this type of prior-art ultrasonic machine for supplying high-frequency electric energy to the piezoelectric transducer generally adopts a constant amplitude method in which frequency change is automatically followed up in feedback control. The present invention is made to overcome problems which remain unsolved by such prior-art ultrasonic machines. The conventional constant amplitude method will be explained below with reference to FIG. 2.
In FIG. 2, 26 generally indicates an ultrasonic vibration unit comprising a piezoelectric transducer 12 and a tool 14 mounted thereon via a horn 16. The piezoelectric transducer 12 is provided with a vibration detector 28 comprising an electrostrictive element. A high frequency voltage generated by a voltage controlled oscillator 30 which is provided in a high frequency oscillator 58 is sent via a waveform shaping circuit 32 and a variable amplitude amplifier 34 to a power amplification circuit 36 where the high frequency voltage is electrically amplified. The amplified high frequency voltage is then input to the piezoelectric transducer 12, which vibrates the ultrasonic vibration unit at a predetermined amplitude. At this time, the vibration detector 28 receives a sine wave voltage proportional to the vibrational speed of the piezoelectric transducer 12. Part of the received sine wave voltage output is fed back to a detection circuit 38.
The detection circuit 38 rectifies the feedback voltage, from the vibration detector 28 compares the feedback voltage with a reference voltage at an input line 40, and feeds back the voltage difference between the feedback voltage and the reference voltage at the input line 40 to the variable amplitude amplifier 34 via an amplifier 42 and a time constant circuit 44. In this way, constant amplitude control of the piezoelectric transducer 12 corresponding to the reference voltage is performed at all times regardless of the load applied to the tip of the tool 14 during ultrasonic machining.
Furthermore, after being partly fed backed to a zero point detection circuit 46 which is provided in the high frequency oscillator 58, the sine wave voltage from the vibration detector 28 is sent to a phase comparison circuit 50 where it is compared with the high frequency voltage which has been phase-controlled by a phase control circuit 48. Direct voltage corresponding to the phase difference is positively fed back to the voltage controlled oscillator 30 via a DC amplifier 52, thus matching the oscillation frequency with the resonance frequency of the piezoelectric transducer 12.
Depending on the type of ultrasonic machining, the tool 14 may have to be extremely small in size in comparison with the piezoelectric transducer 12 and the horn 16. In such a case, the greater the load applied to the tip of the tool 14 becomes, the smaller the amplitude of the tool in contact with the work becomes even though the amplitude of the high frequency supplied to the piezoelectric transducer 12 is controlled to be constant. This results in deterioration of machining efficiency, and of the chip disposal rate; and, machining failure in the worst case.
Because the tool 14 tends to slip on the work in the initial stage of ultrasonic machining, the high frequency amplitude supplied to the piezoelectric transducer 12 needs to be small or the feed speed of the tool 14 needs to be slow. If machining is started with a large amplitude, various problems will arise including a shortened life of the tool 14, unnecessary exothermic effects, and noise as well as the slippage of the tool 14 mentioned above.
In addition, deeper cutting by the tool 14 necessitates a larger amplitude of the tool 14 so as to facilitate chip disposal. Also, fine adjustment of amplitude may be necessary to suit sharpness of the tool 14. The operator of ultrasonic machines has depended on his/her skill and experience to make these adjustments by changing the reference voltage at the input line 40 so that the most appropriate amplitude is obtained for different types of machining.